orinoco: more reliable scan handling

Bring scan result handling more in line with drivers like ipw.  Scan
results are aggregated and a BSS dropped after 15 seconds if no beacon
is received.  This allows the driver to interact better with userspace
where more than one process may request scans or results at any time.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>